Dosimeter And Associated Method Of Measuring Radiation

ABSTRACT

A dosimeter and an associated method for detecting radiation are provided. A dosimeter includes a complementary pair of transistors, such as a first transistor that is doped in accordance with a first conductivity type, such as an n-doped metal oxide semiconductor field effect transistor (MOSFET) and a second transistor that is doped in accordance with a second conductivity type, different than the first conductivity type, such as a p-doped MOSFET. The first and second transistors may be configured to generate respective outputs that shift in opposite directions in response to radiation. The dosimeter may also include a circuit element configured to determine a measure of the radiation based upon a difference between the respective outputs of the first and second transistors. The circuit element may include an amplifier configured to amplify the difference between the respective outputs of the first and second transistors.

FIELD

Embodiments of the present disclosure relate generally to dosimeters andassociated methods of measuring radiation and, more particularly todosimeters that utilize transistors having different conductivity typesin order to determine a measure of the radiation.

BACKGROUND

It is desirable to be able to detect and measure ionizing radiation invarious instances. For example, space and air vehicles may desire todetect and measure radiation to assist, for example, in the avoidance ofradiation exposure. Additionally, terrestrial applications, such asvarious security applications, may desire to detect and measureradiation, either to assist in the avoidance of radiation exposure orotherwise.

Geiger counters are conventionally utilized to detect and measureradiation. While functional, Geiger counters may be relatively expensivewhich may curtail their widespread use. In addition, Geiger counters maybe considered relatively sizeable with their size also serving to limitthe applications that may be effectively served by Geiger counters.

RADiation-sensitive Field Effect Transistors (RADFETs) have also beendeveloped to detect radiation. A RADFET is a p-channel enhancement metaloxide semiconductor field effect transistor (MOSFET) that isspecifically designed to respond to doses of ionizing radiation. Furtherdetails regarding RADFETs are provided, for example, by Holmes-Siedle,Ravotti and Glazer, “The Dosimetric Performance Of Radfets In RadiationTest Beams”, IEEE Nuclear and Space Radiation Effects Data Workshop,July 2007. RADFETs generally have a relatively large starting voltage.Upon exposure to radiation, the output of a RADFET shifts or alters fromthe relatively large starting voltage by a relatively small amount. Inan effort to facilitate the measurement of the amount by which theoutput of a RADFET has shifted and, in turn, the amount of radiation towhich the RADFET was exposed, the output of a RADFET may be amplified.The amplified output is limited, however, to a value no greater than thesupply voltage utilized for the amplifier. Since the amplification ofthe output of the RADFET amplifies not only the change in the RADFET'soutput that is occasioned as a result of the exposure to radiation, butalso relatively large starting voltage, the output of a RADFET may notbe amplified as much as desired in some applications.

As a result of the relatively small change in the output of a RADFETupon the exposure to radiation and the limitations upon theamplification of the change, the sensitivity with which RADFETs maydetect incident radiation may be somewhat limited and, at least for someapplications, may be less than desired. The relatively small magnitudeof the change in the output of a RADFET in response to the exposure toradiation may also require relatively complex detection and measurementcircuitry to be required in order to detect changes in the output of aRADFET in response to exposure to radiation. This detection andmeasurement circuitry may disadvantageously increase the cost of adosimeter that relies upon RADFET technology.

It may therefore be desirable to develop an improved dosimeter that mayhave a relatively small form factor and may be more economical, whileoffering improved sensitivity relative to at least some of the existingdosimeters.

SUMMARY

An improved dosimeter and an associated method for detecting radiationare therefore provided according to embodiments of the presentdisclosure. In one embodiment, the dosimeter and associated methodprovide the measure of the incident radiation in such a manner that theincident radiation can be measured with enhanced sensitivity. Inaddition, the dosimeter and associated method of one embodiment areconfigured such that the dosimeter may have a relatively small formfactor and may be quite economical, thereby facilitating more widespreadusage.

In one embodiment, a dosimeter is provided that includes a firsttransistor that is doped in accordance with a first conductivity type,such as an n-doped metal oxide semiconductor field effect transistor(MOSFET). The dosimeter of this embodiment also includes a secondtransistor that is doped in accordance with a second conductivity type,different than the first conductivity type. For example, the secondtransistor may be a p-doped MOSFET. The first and second transistors areconfigured to generate respective outputs that shift in oppositedirections in response to radiation. The dosimeter also includes acircuit element configured to determine a measure of the radiation basedupon a difference between the respective outputs of the first and secondtransistors. In one embodiment, the circuit element includes anamplifier configured to amplify the difference between the respectiveoutputs of the first and second transistors. The dosimeter of oneembodiment may also include a constant current source for providing aconstant level of current to the first and second transistors.

The dosimeter may also include a resistive element in electricalcommunication with one of the first and second transistors. Theresistive element of this embodiment is configured to alter the outputof the transistor with which the resistive element is in electricalcommunication such that the respective outputs of the first and secondtransistors have a predefined relationship, such as by being equal, inthe absence of radiation. The dosimeter of one embodiment may alsoinclude a temperature sensitive circuit in electrical communication witha respective one of the first and second transistors. The temperaturesensitive circuit of this embodiment is configured to alter the outputof the respective transistor with which the temperature sensitivecircuit is in electrical communication. In this embodiment, themagnitude of the changes in the output of the respective transistor willmatch the magnitude of the changes in the output of the other transistoras the temperature of the first and second transistors changes.

In another embodiment, the method of measuring radiation is provided.The method provides a first transistor that is doped in accordance witha first conductivity type, such as an n-doped MOSFET. The method alsoprovides a second transistor that is doped in accordance with a secondconductivity type, different than the first conductivity type, such as ap-doped MOSFET. The method of this embodiment receives radiation withthe first and second transistors and then generates respective outputsfrom the first and second transistors that shift in opposite directionsin response to the radiation. The method also determines a measure ofthe radiation based upon a difference between the respective outputs ofthe first and second transistors. In one embodiment, the determinationof the measure of the radiation includes amplifying the differencebetween the respective outputs of the first and second transistors. Themethod of one embodiment may also provide a constant level of current tothe first and second transistors.

The method of one embodiment may also provide a resistive element inelectrical communication with one of the first and second transistors.The method of this embodiment may also alter the output of thetransistor with which the resistive element is in electricalcommunication such that the respective outputs of the first and secondtransistors have a predefined relationship in the absence of radiation.Additionally, or alternatively, the method of one embodiment may providea temperature sensitive circuit in electrical communication with therespective one of the first and second transistors. The method of thisembodiment may also alter the output of the respective transistor withwhich the temperature sensitive circuit is in electrical communication.As such, a magnitude of the changes in the output of the respectivetransistor will match the magnitude of the changes in the output of theother transistor as the temperature of the first and second transistorschanges.

By providing a dosimeter and an associated method in which the measureof radiation is based upon a difference between the respective outputsof a pair of transistors, the measure of radiation that is generated inresponse to the exposure to radiation may be greater than in someconventional dosimeters so as to provide for enhanced sensitivity.Additionally, by basing the measure of the radiation upon the differencebetween the respective outputs of a pair of transistors and not upon arelatively small change that is made to a relatively large startingvoltage, the measure of radiation may be more substantially amplified,thereby further increasing the magnitude of the measure of the radiationand further enhancing the sensitivity of the dosimeter and theassociated method.

The features, functions and advantages that have been discussed can beachieved independently in various embodiments of the present disclosureor may be combined in yet other embodiments, further details of whichcan be seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the disclosure in general terms, reference willnow be made to the accompanying drawings, which are not necessarilydrawn to scale, and wherein:

FIG. 1 is a diagram of a dosimeter in accordance with one embodiment ofthe present disclosure;

FIG. 2 is a flow chart illustrating the operations performed inaccordance with a method of one embodiment of the present disclosure;and

FIG. 3 is a graphical representation of the output of a dosimeter inaccordance with one embodiment of the present disclosure in response todifferent levels of radiation exposure at different temperatures.

DETAILED DESCRIPTION

The present disclosure now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the disclosure are shown. Indeed, the disclosure may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

According to embodiments of the present disclosure, a dosimeter 10 isprovided in order to provide a measure of the radiation to which thedosimeter is exposed. As such, the dosimeter may be employed in a widevariety of applications including onboard space vehicles and airvehicles in order to detect potential exposure to radiation by crewmembers, passengers, cargo, or the like. Additionally, the dosimeter ofembodiments of the present disclosure may be utilized in a variety ofterrestrial applications to detect the potential exposure to radiation.

As shown in FIG. 1, the dosimeter 10 includes first and secondtransistors 12, 14. The first and second transistors may be a pair ofcomplementary transistors and, as such, may be doped in accordance withfirst and second conductivity types, respectively. In this regard, thefirst and second conductivity types are different. As such, the firsttransistor 12 may be an n-doped transistor, while the second transistor14 may be a p-doped transistor. The dosimeter may utilize a variety ofdifferent types of transistors, although the particular dosimetergenerally includes first and second transistors of the same generaltype, albeit of different conductivity types. In the illustratedembodiment, for example, the first and second transistors are MOSFETs.In this embodiment, the gate oxide layer of the MOSFETs is generallysufficiently thick to provide the sensitivity desired for theapplication, since the sensitivity of the MOSFET will generally varydirectly with the thickness of the gate oxide layer with a thicker oxidelayer providing enhanced sensitivity and a thinner oxide layer providingdecreased sensitivity. Also, the gate oxide layer of the MOSFETs of thisembodiment is generally sufficiently thick to avoid annealing at roomtemperature due to the tunneling of electrons into the gate oxide layersince such annealing may otherwise disadvantageously cause the signal tofade over time. In one embodiment, for example, the gate oxide layer mayhave a thickness of at least 100 Angstroms. However, the gate oxidelayer may have other minimum thicknesses in other embodiments, if sodesired.

The dosimeter 10 may also include constant current sources 16, one ofwhich is associated with each of the first and second transistors 12, 14in order to provide a flow of current through a respective transistor toground. As such, the transistors are generally configured to be inparallel between the constant current sources and ground as shown inFIG. 1. The constant current sources are configured to provide a flow ofcurrent, such as 390 microamps, that is sufficient to bias therespective transistor such that the transistors remain operationalwithin the linear region in response to exposure to the anticipatedlevels of radiation so as to insure that the respective transistorsappropriately respond to radiation by altering their outputs withoutbecoming saturated. The constant current sources of one embodiment arealso configured to provide a flow of current sufficient to bias thetransistors to an operational regime in which the transistors will havea reduced temperature variation.

In the absence of radiation, each transistor provides predefined outputin response to the current flow therethrough as provided by therespective constant current source 16. As described below, the first andsecond transistors 12, 14 of the dosimeter 10 of one embodiment areconfigured such that their outputs have a predefined relationship in theabsence of radiation, such as with the output of each transistor havingthe same value, such as zero, or otherwise being within the predefinedrange of one another. In response to radiation, however, the output ofeach transistor changes. By utilizing a pair of complementarytransistors that have opposite conductivity types, the outputs of thefirst and second transistors change or vary in response to radiation inopposite manners. For example, the output of the first transistor mayincrease in response to the exposure to radiation, while the output ofthe second transistor may decrease in response to the exposure to thesame radiation. In the embodiment in which both transistors have anoutput of zero volts in the absence of radiation, the first transistormay provide a positive voltage in response to the exposure to radiation,while the second transistor may provide a negative voltage in responseto the exposure to the same radiation.

The dosimeter 10 of one embodiment also includes a circuit elementconfigured to determine a measure of radiation based upon a differencebetween the respective outputs of the first and second transistors 12,14. In the illustrated embodiment, the circuit element is embodied as anamplifier 18 that receives the outputs of the first and secondtransistors and that amplifies the difference therebetween. By utilizinga pair of transistors having outputs that vary in opposite directions inresponse to the exposure to radiation and by determining a measure ofthe radiation based upon the difference between the respective outputsof the first and second transistors, the measure of the radiation thatis generated by the dosimeter of embodiments of the present disclosuremay be larger than that generated by a RADFET when exposed to the samelevel of radiation since the RADFET generates an output based upon thechange in the output of a single transistor.

Additionally, the voltage level that is being amplified is a voltagelevel representative of the difference between the respective outputs ofthe first and second transistors 12, 14. Although this difference isgenerally larger than the change in the output of a RADFET in responseto exposure to the same radiation, this difference remains a relativelysmall voltage since the voltage difference is not built upon arelatively large starting voltage as in the case of a RADFET. As such,the difference may be substantially amplified by the amplifier 18, suchas a differential voltage amplifier embodiment, for example by a lownoise, low offset operational amplifier. In this regard, the relativelylarge amount of amplification may be provided by the amplifier 18 of oneembodiment since the relatively small difference that is being amplifiedcan be substantially amplified without causing the output of theamplifier to reach or attempt to exceed the magnitude of the supplyvoltage, which effectively defines the maximum amount of amplificationprovided by the amplifier. In this regard, the amplifier may include a 0volt and a 5 volt supply voltage such that a relatively small differencevalue such as 4 mV, can be multiplied by a predetermined amplificationfactor of 1000 without exceeding the magnitude of the supply voltage,e.g., 5 volts. In contrast, the output of a RADFET that is amplified isnot only the relatively small change that occurred in the output inresponse to the exposure to radiation, but also the relatively largestarting voltage. As such, the amount of amplification of the output ofa RADFET is generally substantially more limited than that provided inaccordance with embodiments of the present disclosure.

As noted above, the outputs of the first and second transistors 12, 14in the absence of radiation are generally the same or at least within apredefined range of one another. In order to facilitate the outputs ofthe first and second transistor having a predefined relationship, suchas being identical or at least within a predefined range of one anotherin the absence of radiation, the dosimeter 10 of one embodiment mayinclude a resistive element 20, such as a potentiometer, in electricalcommunication with one of the first and second transistors. In theillustrated embodiment, for example, a potentiometer is connected inline between the constant current source 16 and the n-MOS transistor.The resistive element is configured to alter the output of thetransistor with which the resistive element is in electricalcommunication. As such, the resistant value provided by thepotentiometer may be adjusted until the respective outputs of the firstand second transistors in the absence of radiation have a predefinedrelationship. For example, the resistive value provided by thepotentiometer may be adjusted until the respective outputs of the firstand second transistors are identical to one another in the absence ofradiation.

By utilizing a complementary pair of transistors, the dosimeter 10provides for first order temperature compensation. However, thedosimeter of one embodiment of the present disclosure may include atemperature sensitive circuit 22, such as a thermistor circuit, asdepicted in FIG. 1 to provide for second order temperature compensation,thereby further improving the performance of the dosimeter. In theillustrated embodiment, for example, the dosimeter 10 also includes atemperature sensitive circuit in electrical communication with arespective one of the first and second transistors 12, 14. While thedosimeter 10 of the illustrated embodiment depicts the temperaturesensitive circuit to be in electrical communication with the opposite orother transistor from that with which the resistive element 20 is inelectrical communication, both the resistive element and the temperaturesensitive circuit could be in electrical communication with the sametransistor. Additionally, the dosimeter may include only one of theresistive element or the temperature sensitive circuit or may includeboth the resistive element and the temperature sensitive circuit asdepicted in the embodiment of FIG. 1. As shown in the example of FIG. 1,the temperature sensitive circuit may be positioned in line between theconstant current source 16 and the p-MOS transistor. The temperaturesensitive circuit is configured to alter the output of the respectivetransistor with which the temperature sensitive circuit is in electricalcommunication. In particular, the temperature sensitive circuit isconfigured to provide different predefined resistance levels atdifferent temperatures such that the magnitude of changes in the outputof the respective transistor with which the temperature sensitivecircuit is in electrical communication match, e.g., equal, of magnitudeof the changes in the output of the other transistor as the temperatureto which the first and second transistors are exposed changes. Althoughthe temperature sensitive circuit facilitates the matching orequalization of the magnitude of the outputs of the pair of transistorsas the temperature changes, the outputs of the pair of transistors stillvary in opposite directions, generally by equal but opposite amounts, inresponse to the exposure to radiation.

In operation, a dosimeter 10 may be provided with a complementary pairof transistors and with a constant level of current being provided toboth the first and second transistors 12, 14. See operations 30 and 32of FIG. 2. The dosimeter and, in particular, the first and secondtransistors may then be exposed to radiation and, in response, the firstand second transistors may generate respective outputs that shift inopposite directions in response to the radiation. See operations 38 and40 of FIG. 2. The measure of the radiation may then be determined basedupon a difference between the respective outputs of the first and secondtransistors. See operation 42. In this regard, the measure of theradiation may be determined by amplifying the difference between therespective outputs of the first and second transistors.

In order to controllably establish the respective outputs of the firstand second transistors 12, 14 in the absence of radiation, a resistorcircuit 20 may be provided in electrical communication with one of thetransistors such that the output of the transistor with which theresistive element is in electrical communication may be altered in orderfor the respective outputs of the transistors to have a predefinedrelationship, such as by being equal or being within a predefined rangeof one another, in the absence of radiation. See operation 34 of FIG. 2.Additionally, or alternatively, a temperature sensitive circuit 22 mayalso be provided that is in electrical communication with the respectiveone of the first and second transistors such that the output of therespective transistor with which the temperature sensitive circuit is inelectrical communication is altered as the temperature to which thetransistors are exposed varies. See operation 36 of FIG. 2. As such, themagnitude of the changes in the output of the respective transistor maymatch the magnitude of the changes in the output of the other transistoras the temperature to which the first and second transistors are exposedchanges.

By way of example, a dosimeter 10 in accordance with one embodiment ofthe present disclosure that includes a temperature sensitive circuit 22for providing second order temperature compensation was evaluated over arange of temperatures from about −20° C. to about 50° C. In this regard,the output of the dosimeter over this temperature range in the absenceof radiation (designated Prerad), in response to exposure to 40 rads ofradiation and in response to 80 rads of radiation was evaluated and isdepicted in FIG. 3. As illustrated, the dosimeter of this embodiment hada sensitivity of approximately 12.5 mV/rad over this relatively largetemperature range. Although the dosimeter could have been constructed soas to have increased sensitivity, such as 150 mV/rad or more, thedosimeter of this embodiment still had a substantially greatersensitivity than a number of conventional RADFET dosimeters which have asensitivity generally on the order of about 1 mV/rad. Moreover, theutilization of relatively inexpensive transistors, such as p-MOS andn-MOS transistors, provide for a relatively small form factor and arelatively economical design, while still providing enhanced performanceas shown, for example, in FIG. 3.

Many modifications and other embodiments of the disclosure set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the disclosure is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A dosimeter comprising: a first transistor that is doped inaccordance with a first conductivity type; a second transistor that isdoped in accordance with a second conductivity type, different than thefirst conductivity type, wherein the first and second transistors areconfigured to generate respective outputs that shift in oppositedirections in response to radiation; and a circuit element configured todetermine a measure of the radiation based upon a difference between therespective outputs of the first and second transistors.
 2. A dosimeteraccording to claim 1 wherein the circuit element comprises an amplifierconfigured to amplify the difference between the respective outputs ofthe first and second transistors.
 3. A dosimeter according to claim 1further comprising a resistive element in electrical communication withone of the first and second transistors, the resistive elementconfigured to alter the output of the transistor with which theresistive element is in electrical communication such that therespective outputs of the first and second transistors have a predefinedrelationship in the absence of radiation.
 4. A dosimeter according toclaim 1 further comprising a temperature sensitive circuit in electricalcommunication with a respective one of the first and second transistors,wherein the temperature sensitive circuit is configured to alter theoutput of the respective transistor with which the temperature sensitivecircuit is in electrical communication such that a magnitude of changesin the output of the respective transistor match a magnitude of changesin the output of the other transistor as a temperature of the first andsecond transistors changes.
 5. A dosimeter according to claim 1 furthercomprising a constant current source for providing a constant level ofcurrent to the first and second transistors.
 6. A dosimeter according toclaim 1 wherein the first transistor comprises an n-doped metal oxidesemiconductor field effect transistor (MOSFET) and the second transistorcomprises a p-doped MOSFET.
 7. A dosimeter comprising: a firsttransistor that is doped in accordance with a first conductivity type; asecond transistor that is doped in accordance with a second conductivitytype, different than the first conductivity type, wherein the first andsecond transistors are configured to generate respective outputs thatshift in opposite directions in response to radiation; a resistiveelement in electrical communication with one of the first and secondtransistors, the resistive element configured to alter the output of thetransistor with which the resistive element is in electricalcommunication such that the respective outputs of the first and secondtransistors have a predefined relationship in the absence of radiation;a temperature sensitive circuit in electrical communication with arespective one of the first and second transistors, wherein thetemperature sensitive circuit is configured to alter the output of therespective transistor with which the temperature sensitive circuit is inelectrical communication such that a magnitude of changes in the outputof the respective transistor match a magnitude of changes in the outputof the other transistor as a temperature of the first and secondtransistors changes; and a circuit element configured to determine ameasure of the radiation based upon a difference between the respectiveoutputs of the first and second transistors.
 8. A dosimeter according toclaim 7 wherein the circuit element comprises an amplifier configured toamplify the difference between the respective outputs of the first andsecond transistors.
 9. A dosimeter according to claim 7 furthercomprising a constant current source for providing a constant level ofcurrent to the first and second transistors.
 10. A dosimeter accordingto claim 7 wherein the first transistor comprises an n-doped metal oxidesemiconductor field effect transistor (MOSFET) and the second transistorcomprises a p-doped MOSFET.
 11. A method of measuring radiationcomprising: providing a first transistor that is doped in accordancewith a first conductivity type and a second transistor that is doped inaccordance with a second conductivity type, different than the firstconductivity type; receiving radiation with the first and secondtransistors; generating respective outputs from the first and secondtransistors that shift in opposite directions in response to theradiation; and determining a measure of the radiation based upon adifference between the respective outputs of the first and secondtransistors.
 12. A method according to claim 11 wherein determining themeasure of the radiation comprises amplifying the difference between therespective outputs of the first and second transistors.
 13. A methodaccording to claim 11 further comprising: providing a resistive elementin electrical communication with one of the first and secondtransistors; and altering the output of the transistor with which theresistive element is in electrical communication such that therespective outputs of the first and second transistors have a predefinedrelationship in the absence of radiation.
 14. A method according toclaim 11 further comprising: providing a temperature sensitive circuitin electrical communication with a respective one of the first andsecond transistors; and altering the output of the respective transistorwith which the temperature sensitive circuit is in electricalcommunication such that a magnitude of changes in the output of therespective transistor match a magnitude of changes in the output of theother transistor as a temperature of the first and second transistorschanges.
 15. A method according to claim 11 further comprising providinga constant level of current to the first and second transistors.
 16. Amethod according to claim 11 wherein providing the first transistorcomprises providing an n-doped metal oxide semiconductor field effecttransistor (MOSFET) and wherein providing the second transistorcomprises providing a p-doped MOSFET.